Types of Transmission Line Structures and Their Components

The United States power grid is made up of more than half a million miles of transmission lines, delivering power to more than 160 million customers, according to the National Renewable Energy Laboratory. Understanding the basics of transmission systems is crucial if you are working in the utility-scale solar sector, because that’s how many large solar power plants interconnect to the grid. 

Transmission line structures, which support high-voltage conductors, are defining features of power transmission systems. In this excerpt from the Introduction to Electric Transmission course, Tim Taylor examines various support structures designed for 115-500 kV lines and explains their essential components.

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This is a nice figure because it shows a lot of different structure types in one place. I just wanted to hit a couple high points here. In this figure, in general, you’ve got lower voltages up top and then you’ve got higher voltages down at the bottom.

Up at the top, I see some 115 kV and 230 kV structures, and then down at the bottom, they go up to 500 kV. As you get up into 500 kV, it’s actually showing the towers are more substantial. They are taller. At 115 kV, you don’t need as much support. Up on the top line, we do see a couple H-frames, and I don’t talk about those in more detail here, but the H-frames in this particular they’re wood, which is actually the transmission line that runs about a quarter mile away from my house is wood H-frame at 230 kV. This one on the left is 115 kV, and the one on the right is 230 kV. 

Also, I wanted to mention that these different structures that are shown right here, they will either be carrying one transmission line with three conductors or two transmission lines, in which case you will see the insulators for actually six conductors, because when you’re looking at two three-phase lines, then you’ve got six conductors. These, the ones that are shown here, are either single-line or double-line construction. 

Of course, we see the H-frames and then we see some monopole designs with just one pole. Then, there are a number of lattice structures/lattice towers that are shown in this picture.  

We actually looked at this tower in a previous lecture, so I will not go into great detail here, but a couple of things I didn’t mention was that structures oftentimes will have a cross arm, and the cross arm supports the insulators which then support the conductors themselves.

We had talked about the shield wires before for lightning protection, and the shield wires are connected to ground wires and the ground wires are connected to ground. So they actually run to the base of the structure and they’ll be typically ground rods that the crews drill into the ground to establish that ground from the energized conductors. 

Ground is a funny concept in electrical engineering and electrical systems. You can think of it as being not energized; although, that’s not always the case. Ground is meant to be neutral or not energized. Like these phase conductors, they have 230 kV or 138 kV energized conductors, and the ground is not supposed to be energized. It’s supposed to be safe.